Is it actually in your back, or your butt? Yes, everything's connected, but here's my thought:
I have (self-diagnosed) piriformis syndrome, and the pain (for me) centers in my foot, knee, and butt, although there are times it makes its way up to my lower back, but that pain is very definitely muscular rather than the nerve. Sitting, or standing aggravate it, as does driving. (Walking and yoga targeting the hips help enormously.) Basically, I have to avoid staying in a position where I'm leaning forward from the hips a lot (so, no riding for right now, and frequent stretch breaks if I'm sitting in an office chair or in class for a long period of time). I'd rule out a slipped disk before doing anything else, but I'd consider seeing a PT for the latter two issues if I were you.